Filled cell electrodialysis, also called electrodeionization ("EDI"), is a recognized means for achieving high levels of desalination of water. The process involves an incorporation of an ion exchange material (such as an ion exchange resin), in an aqueous feed (or an ion depletion) compartment formed in a gasket positioned between anion and cation exchange membranes. The aqueous feed stream that needs to be desalted flows in the ion depletion compartment.
A secondary aqueous stream is circulated in the adjacent concentration compartments. Under a direct current driving force, the ions in the feed stream are transported from the ion depletion to the concentration compartments. The process is similar to conventional electrodialysis, except that the use of ion exchange material in the ion depletion compartment provides an increased electrical conductivity and facilitates higher levels of desalination, with a higher overall current efficiency. The EDI process is particularly well suited for desalting aqueous solutions of low to moderate ion content.
The EDI apparatus and its applicability to obtaining high purity water are detailed in a number of U. S. patents, such as: U.S. Pat. Nos. 4,066,375; 4,203,976; 4,243,976; 4,249,422; 4,465,573; 4,632,745; 4,727,929; 4,871,431; 4,925,541; 4,931,160; 4,956,071; 4,969,983; 5,066,375; 5,116,509; 5,154,809; 5,120,416; 5,203,976; 5,292,422; 5,308,466; 5,316,637 and 5,503,729.
Also, a related disclosure is found in my two recently filed U.S. patent applications Ser. No. 08/784,050 and Ser. No. 08/785,648, both filed Jan. 17, 1997 (pending). The gaskets or chambers and apparatuses used in the EDI units disclosed in these patents and applications have a variety of designs.
The major application for EDI to date has been in the production of high purity water from suitably pre-purified softened aqueous feed streams having a relatively low initial salt content and electrical conductivity. Specific documents that describe the current state of the art are:
Ganzi, G. C.; "Electrodeionization for High Purity Water Production", in a paper presented at the 1987 AIChE Summer National Meeting, August 1987; PA0 Ganzi, G. C. et al, "High Purity Water by Electrodeionization: Performance of the Ionpure.RTM. Continuous Deionization System, Ultrapure Water," 4(3), April 1987. PA0 Ganzi, G. C., et al, "Production of Pharmaceutical Grades of Water using Ionpure Continuous Deionization Post-Reverse Osmosis", in a paper presented at the ICOM meeting, Chicago, Ill., 1990 PA0 Ganzi, G. C., et al, "Water Purification and Recycling using the Ionpure CDI Process", presented at the AIChE Summer National Meeting, Pittsburgh, Pa., August 1991 PA0 Hernon, B. P., et al, "Progress Report: Application of Electrodeionization in Ultrapure Water Production", in a paper presented at the 56 Annual Meeting International Water Conference, Pittsburgh, Pa., October 1995. PA0 Allison, R. P., "The Continuous Electrodeionization Process", in a paper presented at the American Desalting Association 1996 Biennial Conference & Exposition, Monterey, Calif., August 1996. PA0 The streams encountered in the commercial plants have a high sugar content, 40-60 wt %, and hence a high osmotic pressure. In effect, this rules out the use of an RO (reverse osmosis) pre-treatment process for removing the ions from such solutions. The RO process is inappropriate for this application since the desired product is desalted syrup and is not desalted water. PA0 Softening the feed to the EDI unit, via an ion exchange, would require raising the pH of the sugar solution. This is not a viable option since it would exacerbate the bacterial contamination problem. An additional factor is that sugar solutions are unstable in alkaline conditions. PA0 The high levels of the divalent ions in relation to the total ion load, coupled with the large stream flows encountered in commercial plants would make an upstream softening step expensive and impractical.
The EDI process can be visualized as occurring in two regimes, depending upon the ionic content of the aqueous feed to be desalted and certain operational parameters such as flow rate, current input etc. At higher ionic concentrations and when operating below a limiting current density, the process operates somewhat as the conventional electrodialysis process operates.
In this first regime (herein termed "Regime I"), the ion exchange material in the feed stream or ion depleting compartments primarily increases the electrical conductivity of the feed stream or the ion depleting loop, thereby facilitating significant levels of de-ionization. The presence of ion exchange material in the feed loop lowers the electrical resistance for ion transport, thereby allowing significantly higher levels of de-ionization in the EDI than would be feasible via conventional electrodialysis.
Often, the ion exchange material is a mixture of cation and anion exchange resins in the feed stream (or ion depletion) compartment. This material greatly facilitates the transport of the cations and anions in the feed solution to the surfaces of the cation and anion exchange membranes, respectively. As a result, the overall electrical conductivity in the feed loop increases, resulting in an increased current transport for a given applied voltage. At the same time, the current efficiency--the ratio of the equivalents of ions transported per faraday of current input--for the EDI process also remains high; usually &gt;0.5. The extent of water splitting is quite small, even when the conductivity of the desalted feed is in the range of 10 to &lt;1 .mu.S/cm.
In principle, conventional electrodialysis (i.e., without the use of ion exchange material in the feed loop) can operate in this regime. However, this operation has been deemed unsuitable from an economic standpoint because of the low current throughput (i.e. because of the poor conductivity in the depleted feed loop) and the large membrane areas that are required which leads to a high capital cost.
A limiting current density is reached in EDI when sufficient amounts of ions are not available in the resin and membrane boundary layers for current transport, even at lower ion concentrations, as compared to concentrations in conventional electrodialysis. In this mode of operation (herein termed "Regime II"), the application of electrical current results in the dissociation or splitting of water molecules into hydrogen (H.sup.+) and hydroxyl (OH.sup.-) ions. At least in part, these ions displace the ions present in the ion exchange resin, in effect regenerating the resin material. The displaced ions are transported out of the feed loop, across the ion exchange membranes and into the concentrate loop, thus producing highly desalted water. It is in this regime (termed "Regime II"), that the EDI process has found the major commercial use, i.e., production of high purity water with a conductivity of 0.055-0.5 .mu.S/cm or a resistivity of 2-18.times.10.sup.6 ohm cm (2-18 Meg-ohm).
An adequate pretreatment of the feed water is an essential pre-requisite to the reliable long term operation of the EDI unit. The presence of insoluble matter and certain organic foulants in the feed stream may cause a plugging of the cell internals, or an irreversible fouling of the ion exchange material in the feed loop or the ion exchange membranes. Therefore, these insoluble matters and organic foulants must be removed via upstream pretreatment steps.
A further problem in EDI operation is the precipitation of calcium and magnesium ions within the EDI cell due to their poor solubility in the environments found within such cells. An addition of an acid, such as hydrochloric, to control the precipitation of the divalent ions is practiced at times in conventional electrodialysis. However, to date, the addition of such acid has not been used in EDI for producing desalted streams, possibly because the problems associated with the water splitting and the attendant pH shifts could pose serious impediments to a reliable operation of the EDI unit. Also, the addition of acid and its subsequent removal imposes additional reagent costs, as well as the downstream operating costs.
For these reasons, softening of the feed stream is used to remove the calcium and magnesium ions from the feed solutions. However, this pre-treatment process also has some costs and associated process complexities. The softening process is carried out in a column containing a cation exchange resin. When the column is sufficiently loaded with the multivalent cations, it is regenerated by the use of a concentrated salt solution (NaCl) or, by using an acid and a base (usually HCl and NaOH). Either method produces additional waste streams that need to be removed. The softening process often requires a pH adjustment of the feed stream to neutral or alkaline(pH.gtoreq.7) in order to facilitate the removal of calcium and magnesium ions. Furthermore, the softening process replaces the calcium and magnesium values in the feed stream with sodium. In turn, the sodium must be removed via the EDI process.
In many commercial operations, the softened feed stream is subjected to an additional reverse osmosis ("RO") step to further reduce the ion load to the EDI cell stack. In other instances, an RO unit having a high level of rejection of ions (say &gt;98%) may be deployed. If such a unit is used, a separate upstream softening step may be unnecessary. However, this step may result in the production of a substantial volume of a "reject" stream that represents a loss of feed material. Such RO treated feeds to the EDI have a conductivity in the range of only about 5-80 .mu.S/cm. Consequently, the EDI unit operates substantially in the water splitting regime (Regime II).
An improved desalination process is needed that is applicable to streams of low to moderate ion content. One desired process does not require upstream softening or reverse osmosis ("RO") steps with their added costs, attendant process complexities, additional waste generation, and potential yield losses. Improved processes that allow inexpensive, preferably on-site, production of the needed acid for adding to the EDI feed, as well as means for recovering/reusing such acid are also needed.
Particularly, in the production of dextrose and other fermentation products, a number of process applications require high levels of ion removal from aqueous streams. Such feed streams often contain significant levels of calcium and/or magnesium, as well as ions such as sodium, chloride, sulfate, bisulfite, etc. These streams have a relatively high sugar content and density, which in turn increases their viscosity and or osmotic pressure while suppressing their electrical conductivity. A concentrated dextrose solution derived from the hydrolysis of corn starch might, for example, contain 40 ppm calcium, 30 ppm magnesium, .about.100 ppm sodium, and equivalent amounts of anions such as chloride, sulfate etc., impurities such as organic compounds and color bodies; and might have a conductivity of .about.400 .mu.S/cm. Similarly, concentrated high fructose syrup solutions obtained from an enzymatic conversion of purified dextrose might contain .about.45 ppm magnesium (added as a catalyst to assist in the enzymatic conversion operation), 1-10 ppm calcium, .about.100 ppm sodium, equivalent amounts of anions such as chloride, sulfate etc., as well as small quantities of organic acids and have a conductivity of 300-400 .mu.S/cm.
It can be seen that the hardness component of these feed streams is a significant portion of the total ionic load. Therefore, the prior art processes involving softening a feed stream requires a relatively large softening column in front of the EDI cell. Such a two step approach (feed softening+EDI) is expensive and offers no significant improvement over the ion exchange method described below.
It turns out that the desalting of process feed streams, such as dextrose solutions, pose other problems that further negates the economic viability of an upstream softening step. These and other sugar containing solutions are prone to bacterial growth problems and have stability problems at nearly neutral or alkaline pH's.
In order to minimize such contamination problems, the solutions are intentionally acidified. While any acid (such as sulfuric or hydrochloric) may be used to acidify the sugar solutions, the one most commonly used is sulfur dioxide, partly because it has bactericidal properties. In addition, being a weak acid, sulfur dioxide is able to provide a good buffer in the pH range of 2-3 where the sugars exhibit the best stability in solution. About 200-1000 ppm of sulfur dioxide is added during the processing of dextrose and fructose streams. In order to obtain products of satisfactory quality, these streams need to be pre-treated to remove both color and odor, and subsequently to be desalted to provide a final product conductivity of .about.3 .mu.S/cm. Softening such streams prior to EDI is not feasible since this would require an unacceptable upstream pH adjustment step.
At present these sugar solutions are purified by an initial carbon treatment step, followed by a multiple step ion exchange involving alternating cation and anion exchange columns; (see "Diaion.RTM. Manual of Ion Exchange Resins," Volume II, Pages 93-107; by Mitsubishi Kasei Corp., March 1992; Second Printing May 1, 1993). Such exchange columns consume large quantities of acid and alkali (HCl, NH.sub.3 /NaOH) for regeneration, as well as de-ionized water for sugar displacement and rinsing the ion exchange beds. Consequently, the ion exchange route generates large quantities of waste streams that need to be treated and eliminated.
Additionally, the ion exchange process results in some dilution of the original syrup solutions. Substantial amounts of energy and capital have to be expended for re-concentrating the streams. The ion exchange beds used in the desalting process are rather massive; therefore, aside from the required capital costs, one also has operating costs associated with the attrition losses of ion exchange resin. Accordingly, an improved desalting process that overcomes the shortcomings of the ion exchange process is needed.
An electrodeionization process as disclosed in prior art for water desalination/purification is not suitable for use with sugar containing solutions for a number of reasons:
For these reasons, an improved process is needed for purifying such biologically sensitive streams. A process for directly desalting such acidic streams is highly desired.
Also needed are methods for recovering valuable components in the concentrate stream for possible recycle/reuse. In an EDI unit, as with most membrane based processes, a small portion of the component in the feed stream (e.g. sugars cited in the example in the earlier paragraph) would end up in the concentrate stream waste product. Means are needed for recovering such components, if they are valuable, or if their recovery offers substantial environmental benefits. Since magnesium is intentionally added to the high fructose syrup stream in the dextrose isomerization step, a method is needed to recover the magnesium for reuse.